Sheet metal articles can be made by hot blow-forming processes that use complementary forming tools in a press under the pressure of a working gas to stretch-form a preheated sheet metal blank against forming surfaces on the forming tools. Such processes are particularly applicable to forming sheet metal into products of complex three-dimensional curvature. For example, superplastic-forming (SPF) and quick-plastic-forming (QPF) processes are increasingly being used to produce high quality sheet metal products such as automotive body panels. One such process is disclosed in U.S. Pat. No. 6,253,588, entitled “Quick Plastic Forming of Aluminum Alloy Sheet Metal” to Rashid et al., which is assigned to the assignee hereof and which is incorporated by reference herein.
In hot blow-forming, a hot metal sheet is blown against a heated forming surface. Because the sheet is hot, it is quite compliant and takes the shape of the forming surface. Accordingly, SPF and QPF processes are capable of producing automotive body panels with a high degree of resolution with complex shapes and fine surface details.
But, such resolution capability also yields some unintended qualities. For example, surface show-through discontinuities appear when the sheet metal blank is stretch-formed against discontinuities in the forming surfaces on the forming tools. Show-through is the obtrusive communication of surface discontinuities from one side of the sheet metal blank to the other. Such discontinuities are particularly noticeable when the show-through occurs on a Class A surface, which may be generally defined as a surface on a final product that is readily visible to a customer. The sources of show-through and other surface blemishes are many, including uneven lubrication between a sheet metal blank and a tool, contamination or debris on tool surfaces, hair-line cracks in welded zones on tool surfaces, boundaries between an insert and a tool, and gaps between tool elements. The first three sources mentioned above can usually be eliminated by exercising care in maintaining the tooling and equipment. The last two sources mentioned above, however, are impossible to eliminate with maintenance.
Forming tools and forming surfaces thereof may have distinct elements, such as die inserts positioned with a die body for forming special features of the sheet, or lift pads to remove a formed sheet while it is still hot. There may be gaps or discontinuities at the forming surface between adjacent surfaces of the inserts and the body. The gaps are typically on the order of about millimeter and the hot sheet tends to sag into the gap under the pressure of the working gas. This shows up as a depression on a visible surface of the formed part.
To illustrate, FIG. 6 shows a QPF tooling apparatus 10 for forming a workpiece W, wherein the apparatus 10 includes lateral gaps between moving tool elements. The apparatus 10 includes an upper flask tool 12 disposed in vertically reciprocating relation over a lower forming tool 14 with the finish formed workpiece W therebetween. The lower forming tool 14 includes an extraction mechanism 16 for distortion free removal of the formed workpiece W. Extraction pads 18 are provided in recessed portions 20 of the lower forming tool 14 and are simultaneously driven by lift posts 22 and a common lift plate 24 to uniformly strip the delicate workpiece W from a forming surface 26 of the lower forming tool 14.
As best shown in prior art FIG. 7, there is illustrated a tooling apparatus 10′ wherein a lateral gap 28′ results between an outside surface 30′ of a movable forming pad 18′ and a vertical surface 32′ within a respective recess of a forming tool 14′. The gap 28′ may vary in size, but is typically between 0.5 to 2.0 mm. In cases where an insert or forming pad 18′ has been press fit into the forming tool surface 14′, the gap 28′ may be more like a boundary line, but will still produce show-through. Under the high pressures and temperatures of the QPF process, a portion of the formed workpiece W′ sags or draws into the gap 28′, thereby yielding a show-through depression or valley 34′ in an upper surface 36′ of the workpiece W′, and a draw-in bead 38′ projecting beneath a lower surface 40′ of the workpiece W.
Such surface discontinuities are unacceptable for production of high quality exterior body panels. Therefore, the show through valley 34′ may be treated by sanding a large area of the upper surface 36′ of the workpiece W′ around and across the show-through valley 34′ so as to blend the discontinuity with the rest of the workpiece W′. Unfortunately, however, such blending usually just results in shallower but wider visibly depressed regions. So, show-through may be abated by first filling the valley 34′ with a fluid body panel material by hand, then allowing the material to solidify, and finally hand finishing or sanding the material flush with the rest of the upper surface 36′ of the workpiece W′. Unfortunately, however, such treatment is labor-intensive, time consuming, and cost-prohibitive. For all intents and purposes, the draw-in of the workpiece W′ into the gap 28′ results in an effectively unrepairable surface discontinuity in low-cost high-quality production operations.
Thus, the present invention identifies a need for SPF and QPF tooling that does not yield unrepairable show-through surface discontinuities.